1. Field of the Invention
The present invention relates to a spherical phosphor, and a cathode-ray tube, a fluorescent lamp and a radiation intensifying screen using the same.
2. Description of the Related Art
Phosphors for use in cathode-ray tubes, fluorescent lamps, or radiation intensifying screens must have a particle size of several .mu.m to obtain sufficient luminous efficiency when a phosphor is excited by electron beams, ultraviolet radiation or radioactive rays. In order to obtain crystalline particles of several .mu.m in size, phosphor particles are generally synthesized by a solid-phase reaction using a flux. However, the phosphor particles synthesized by the method using the flux are not a completely spherical but near polyhedral shape reflecting crystalline structures of raw materials and/or the resulting phosphors.
When a phosphor screen of a cathode-ray tube, for example, is formed using the polyhedral phosphor, it accompanies the drawback that emission generated by electron beam excitation is not fully utilized as light output from the phosphor screen. More specifically, if the shape of the phosphor particles is near polyhedral, a dense phosphor layer free from void cannot be obtained. In addition, an aluminum backing serving as a reflecting film formed onto the phosphor layer is inferior in smoothness, exhibiting a rough surface. Consequently, the scattering of the emitted light increases causing a loss of light output. In a similar way, if the above phosphor is employed in a fluorescent lamp, emission by the ultraviolet excitation cannot be efficiently utilized since a dense phosphor layer is not obtained.
A color cathode-ray tube is manufactured by, for example, the following method. First, the inner surface of glass is coated with a slurry containing a phosphor and a photosensitive resin, thereby forming a phosphor layer. Subsequently, a desired portion alone is hardened by exposure with ultraviolet radiation. Thereafter, a non-exposed portion of the phosphor layer is washed away. If light scattering of the phosphor layer is large, the ultraviolet ray cannot penetrate deep into the inside portion. As a result, the inside portion is hardly hardened and thus it is hard to form a phosphor layer thick enough to exhibit maximum brightness. If light is scattered excessively, it will be also difficult to make a phosphor layer pattern into a predetermined shape since the portion other than the desired portion is hardened by exposure.
In a cathode-ray tube for use in a projection television, a phosphor layer is generally formed by the following steps. First, phosphor particles are suspended in an aqueous barium acetate solution placed in a glass bulb for cathode-ray tube. To the suspension solution, an aqueous solution of potassium silicate is added and the phosphor particles are allowed to settle onto the inner surface of the glass bulb. Three cathode-ray tubes emitting red, green and blue, respectively, are produced in such steps. Images are magnified by means of three optical lenses individually set in front of the three cathode-ray tubes emitting three colors, and then projected onto a screen. To provide sufficient brightness levels in the screen, a high-power electron gun is used. Even a small defect present on the phosphor layer is magnified clearly on the screen. The dense phosphor screen, therefore, is strongly demanded with the recent tendency toward a high quality image. Furthermore, even under a high load of electron input, it is necessary to minimize a decrease of light output and deterioration.
In most X-ray image intensifiers for use in medical diagnosis and in material examination, an image in the output screen is usually picked up with a TV camera and amplified for observation. To meet such usages, a uniform, dense, and high resolution phosphor screen is required.
The larger the total surface area of phosphor particles contained in the phosphor layer, the more prominent the light scattering. It is therefore desirable that phosphor particles be as spherical as possible. To form spherical phosphor particles, an emulsion method may be employed which is disclosed in B. C. Grabmaier, W. Rossner, J. Leppert; Phys. Stat. Sol. (a) 130, K183 (1992). However, the phosphor obtained by this method is a cluster of fine particles having poor crystalline characteristics, so that the phosphor has to be subjected to refiring. However, the resultant phosphor particles do not always have a completely spherical shape. In addition, their sizes are small. Hence, they are not suitable for use in a cathode-ray tube and a fluorescent lamp.
As another method of forming a spherical phosphor particles is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 62-201989. In this method, a starting phosphor consisting of granulated secondary particles is heated in high-temperature plasma. However, the phosphor formed by this method has drawbacks below: It is difficult to obtain a phosphor having a preferable size by this method. In addition, obtained phosphor has disadvantageous dispersion and adhesion properties. Further, obtained phosphor cannot have a desirable activator concentration suitable for practical phosphor in view of emission color and luminous efficiency.
In a color display, phosphors emitting three colors, red, green, blue are used. A deep red-emitting, bright phosphor is desired for the reason of broadening a color reproduction range as much as possible. A representative red-emitting phosphor for use in a cathode-ray tube, which almost satisfies the above condition is Y.sub.2 O.sub.2 S:Eu. However, the particles of Y.sub.2 O.sub.2 S:Eu are polyhedral, so that Y.sub.2 O.sub.2 S:Eu is not free from a drawback attributed to the light scattering explained above. Hence, it is strongly demanded that the deep red-emitting phosphor having a spherical shape be developed.
A fluorescent lamp for lightning requires not only a sufficient level of brightness, namely luminous efficiency, but the level of making an object color look natural under illumination with a lamp, namely color rendering properties, so that a three-component type fluorescent lamp improved in both luminous efficiency and color rendering properties may be widely used. The three-component type fluorescent lamp can be obtained by mixing the following three phosphors in appropriate amounts and coating the phosphor mixture onto the inner surface of a glass tube:
a blue-emitting phosphor having an emission peak near 450 nm such as a divalent europium-activated barium magnesium aluminate phosphor or a divalent europium-activated barium calcium strontium halophosphate phosphor;
a green-emitting phosphor having an emission peak near 545 nm such as a cerium terbium-activated lanthanum phosphate phosphor or a cerium terbium-activated magnesium aluminate phosphor; and
a red-emitting phosphor having an emission peak near 611 nm such as an europium-activated yttrium oxide phosphor (Y.sub.2 O.sub.3 :Eu).
An average color rendering index, R.sub.a of the fluorescent lamp thus-obtained is as high as 84 to 88. The three-component type fluorescent lamp having this value is excellent in appearing the color of an irradiated object more natural and beautiful. However, the three-component type fluorescent lamp is disadvantageous in that the specific color rendering index R.sub.9 for a red color with high chroma is as low as 20 to 40.
To overcome this problem, the present applicants disclosed a technique in Jpn. Appln. KOKAI Publication No. 5-244878. This technique is that a deep red-emitting europium-activated monoclinic gadolinium oxide (Gd.sub.2 O.sub.3 :Eu) phosphor having an emission peak near 623 nm is blended with the aforementioned three phosphors. By adding the monoclinic Gd.sub.2 O.sub.3 :Eu phosphor to the three phosphors in an amount of 12 wt %, R.sub.9 is successfully improved by 18 points. On the other hand, total luminous flux, however, decreased by 2.4%. To increase R.sub.9 by 10 points, the total luminous flux is inevitably decreased by approximately 1.3%.
Gd.sub.2 O.sub.3 :Eu may belong to a monoclinic crystalline system, as shown in R. C. Ropp: J. Electrochem. Soc., Vol. 112, p. 181 (1965). Gd.sub.2 O.sub.3 :Eu is stable in a cubic system at room temperature, as shown in R. S. Roth et al.: J. Res. National Bureau of Standards, Vol. 64A, p. 309 (1960). In order to obtain a monoclinic system stable at high temperatures, it is necessary to heat Gd.sub.2 O.sub.3 to high temperature of 1200.degree. C. or more, followed by quenching. Therefore, it is difficult to prepare the monoclinic system by a usual method of firing in a crucible.
On the other hand, as shown in Arai et al.:J. Alloys and Compounds, Vol. 192, p. 45 (1993), since a praseodymium-activated monoclinic Gd.sub.2 O.sub.3 has a green emission band which cannot be obtained in the cubic Gd.sub.2 O.sub.3, it may be applied to the usage requiring short persistent green emission. In this case too, it is necessary to overcome the problems in connection with preparing a stable monoclinic system in high temperatures.
In recent years, a fluorescent lamp has been frequently used as a back light for a liquid crystal display. In this case, the fluorescent lamp is used in combination with a reflecting film and a light guide plate and a scattering plate. For saving energy, the luminous efficiency is desired to be as high as possible, when the fluorescent lamp is used in combination with the reflecting film. In a conventional fluorescent lamp, due to low transmittance of a phosphor layer, problematic loss of light occurs during a process in which part of emission light is returned into the fluorescent lamp by a reflecting film, transmits through the lamp and converges in one direction. The tube diameter of the back light for a liquid crystal display is usually set to a significantly small value as compared to that (25 to 35 mm) of a lamp for general lighting, taking brightness and compactness into consideration. In such a fluorescent lamp, phosphor is coated by means of the syringe injection method or the sucking method under reduced pressure, not by the slurry flow method employed for a conventional lamp. In this case, if phosphor particles are aggregated in the slurry and a slurry has poor fluidity, an injection nozzle may be clogged with the aggregated phosphor particles, and a phosphor layer to be formed may have a rough screen.
In the case of a radiation intensifying screen, in order to prevent a decrease in the sensitivity, it may be effective to increase radiation absorption and luminous efficiency by thickening the phosphor layer. However, the thick phosphor layer increases light scattering, with the result that the sufficient sensitivity cannot be obtained. On the other hand, when an average particle size of the phosphor particles used in the phosphor layer is increased, the light scattering can be suppressed but, instead, sharpness of the obtained radiation image is lowered. To obtain an intensifying screen having high sensitivity and creating a sharp radiation image, a method of forming a double-layered phosphor layer by coating phosphor particles having different average particle sizes is used (Jpn. Pat. Appln. KOKAI Publication No. 1-57758). In this method, first, particles are prepared by a wet precipitation and firing method. The obtained particles are classified into two types of phosphor particles having different average particle sizes (e.g., CaWO.sub.4 of 4.2 .mu.m and 9.6 .mu.m in average particle size) by means of a the sedimentation method. To a mixture containing two types of phosphor particles thus-obtained, a binder is added, thereby making a slurry. Thereafter, the slurry is coated on a protection film by means of a knife coater and successively a slurry containing phosphor particles of a smaller average particle size (e.g., CaWO.sub.4 of 4.2 .mu.m in average particle size) alone is coated on the above phosphor layer in the same manner as above. Onto the resultant phosphor layer, a screen base is adhered, thereby forming the intensifying screen. However, this manufacturing process requires too many steps and has a drawback in that it is difficult to set sizes and contents of the phosphor particles since the phosphor particles having different average particle sizes are used. Due to the aforementioned drawbacks, it is difficult to obtain a desired radiation intensifying screen.